ABOVE: © iStock.com, DeanDrobot

Back before the advent of cooking, our early hominid ancestors probably spent a lot of time chewing. And according to a new study published today (August 17) in Science Advances, they likely expended copious amounts of energy doing so. So much energy, in fact, that it may have shaped the evolution of the early human musculoskeletal system, scientists say.  

The study doesn’t delve into the fossil record. Instead, researchers carefully measured how much energy humans burn by chewing, finding that chewing an odorless, tasteless gum elevates the body’s metabolic rate by 10 to 15 percent above baseline. 

“As far as I know, this is the first study that’s ever looked at the energetics of chewing,” says Peter Lucas, an anthropologist at The George Washington University who was not involved in the study but gave feedback to the authors on an earlier draft of the manuscript. “And it really needs to be praised for that reason.”

Estimates for how long humans have been cooking range from 500,000 to 2 million years ago, says Lucas. Our ability to draw out excess calories from meats and starchy foods via heat and tool use “changed the entire energetics of modern humans. And basically, you could say it’s allowed the formation of a very large brain.” In this way, the metabolics of chewing is still “one of the fundamental questions of evolution,” he says, as it may tell us how cooking represents a turning point in humanity’s evolutionary trajectory.

Chewing things over

The study started with a conversation over lunch. Paper coauthor Adam van Casteren, an anthropologist at the University of Manchester in the UK, watched his coworker chew (and continue to chew) a salad, and began wondering “how much more energy would be invested in eating a salad compared to eating a cooked meal?” So he and his colleagues designed an experiment to test just that.

The researchers placed a cohort of volunteers (6 men and 15 women) in a ventilated hood system designed to measure metabolism. The machines—which, according to van Casteren, look “a bit like an astronaut helmet”—measure oxygen uptake and CO2 output. The authors then use the measurements to calculate each participant’s energy expenditure.

Man on a bed with a clear plastic bubble over his head and two tubes coming out.
The ventilated hood system at Maastricht University used to measure oxygen consumed and carbon dioxide produced during activities such as chewing
Amanda Henry

First, volunteers were asked to sit still inside the chamber for 45 minutes, which provided the researchers with a baseline measurement of how much energy they expended at rest. Then, they asked the volunteers to chew an odorless, flavorless gum while inside the chamber for 15 minutes. The researchers tested two levels of gum stiffness with each volunteer. The authors didn’t use foods that subjects could swallow because doing so revs up metabolic processes in the digestive tract, which increases energy expenditure, and they aimed to isolate the cost of chewing.

They found that the soft gum increased the amount of energy the volunteers expended by 10 percent over the initial baseline, while the hard gum boosted energy use by 15 percent. van Casteren says that he “was quite surprised that you saw such a jump between the two gums.” He wondered aloud, “What would be the difference if we actually gave them a hard food or fibrous food or something like that?” 

The tougher gum was significantly softer than most plant-derived foods, the authors noted. And that could mean the measured metabolic cost of chewing is an underestimate. When the researchers used an EMG to record the electrical activity of one of the subject’s masseter jaw muscles in response to chewing the hard or soft gum, they found that the subject chewed the soft gum more frequently but produced more force to chew the hard gum, indicating the amount of force needed is a key driver of mastication’s energetic cost. 

Food for thought 

Lucas says it surprised him that the authors only used chewing gum and not substances that break down as they’re chewed, as the latter would have been more directly comparable to real foods. Of course, the subjects would need to avoid swallowing anything, he says, necessitating that the researchers also calculate “the energetic cost of spitting” to accurately measure the energy expenditure of mastication alone. That might prove tricky, but still, he would like to see future studies try it to increase the relevance of the work—and help elucidate the potential evolutionary significance of chewing.

“Modern humans are quite weird,” says van Castaren. “We don’t chew very much because we cook and process all of our foods before we eat. But our ancestors would have been spending a lot of time chewing.” Based on a back-of-the-envelope calculation of how much time chimpanzees spend chewing, our early hominid ancestors may have been chewing for five or six hours a day, he says, which may have cost them up to 5 percent of the energy they consumed. It’s possible that this drove the evolution of the form or muscle architecture of the jaw, or changes in tooth morphology, van Casteren notes. 

“It’s a new perspective on interpreting changes in the fossil record,” says van Casteren.